Odds are you are beating this summer's heat by reading this in a cool, comfortable place with your air conditioning running. Air conditioning is as familiar as part of our modern world as freeways, jet air travel and gourmet coffee.

Yet most people over the age of 50 grew up in a world in which air conditioning was more a luxury than a part of everyday life. And people in their 70's or beyond can recall a time when air conditioning was reserved for commercial buildings and virtually unheard of in anyone's home.

The invention of air conditioning traces back to early in this century, 1902, when a Brooklyn, NY printing firm found that distortion in the paper caused by excess heat and humidity made it hard to align color images. A clever young engineer named Willis Carrier designed a way to control temperature and humidity for that printing firm. In 1906, Carrier patented that initial device, called "An Apparatus for Treating Air."

Because lack of moisture in the air made fibers hard to weave, textile mills became early customers of Carrier's invention. Many other industrial processes, including pharmaceuticals, tobacco, meat packing, soap, munitions, celluloid film and various hard goods, also benefited from early air conditioning. In 1915 Willis Carrier and a bunch of his friends formed the Carrier Engineering Company to cater to industrial clients. At first they did not manufacture any device called an "air conditioner" per se. They used a variety of components and techniques to control temperature and humidity on a custom design basis. It wasn't until 1922 that Carrier's company began to manufacture what was called a "centrifugal refrigeration machine" that could be installed in a variety of public places.

In 1924, they expanded their horizons from cooling for industrial processes into a human comfort mode. The first comfort cooling system was installed at Detroit's popular J.L. Hudson Department store, where people began to faint from the heat at the store's crowded bargain basement sales. The 1920's also saw movie theaters adopt air conditioning in a competitive race for customers. During the Great Depression of the 1930's, watching a movie in air conditioned comfort was one of the few affordable pleasures available to great masses of the American people. Even into the 1950's, the front of many movie theaters had a familiar blue sign showing a block of ice and advertising "It's cool inside!"

The early days of air conditioning featured large machines available only to large businesses. By the late 1920's demand began to build for compact units that would enable small businesses to compete with the big stores for customer traffic. These "unit air conditioners" gave rise to the development, in 1928, of the "Weathermaker" machine geared to homes. Carrier sponsored an exhibit in the shape of an igloo at the 1939 World's Fair featuring air conditioners suitable for homes and small commercial facilities.

However, first the Great Depression and then World War II retarded the growth of air conditioning for the masses. Residential air conditioning didn't begin to take off until the 1950's. The famous Levittown tract home development in New York included air conditioned homes, and the trend accelerated throughout the building boom in suburban America. In 1955, about 430,000 homes in the United States had central air conditioning. Ten years later the number had grown to an astounding 3 million homes. Ten years after that, by 1975, air conditioning was a part of 36% of all homes built. By 1985, 70% of all new homes had central air conditioning -- about 90% in the South.

In fact, many historians credit the growth of air conditioning as a key factor in the dramatic rise of the southern Sun Belt in population and economic growth during the postwar years. Before air conditioning became widespread, few businesses wanted to locate in the South, whose stifling, sultry summers took a toll on workers and machinery alike. Air conditioning has helped turn the South's warm climate from a handicap into an asset.

And it has made the summertime even more a treat to the residents of the north, who are less acclimated to high temperature and humidity than their southern counterparts. Air conditioning truly ranks as one of the most important inventions of our century for the average citizen. It is a mark of success how much we have come to take it for granted.